LeetCode-in-Java

1. Two Sum

Easy

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

You can return the answer in any order.

Example 1:

Input: nums = [2,7,11,15], target = 9

Output: [0,1]

Explanation: Because nums[0] + nums[1] == 9, we return [0, 1].

Example 2:

Input: nums = [3,2,4], target = 6

Output: [1,2]

Example 3:

Input: nums = [3,3], target = 6

Output: [0,1]

Constraints:

• 2 <= nums.length <= 104
• -109 <= nums[i] <= 109
• -109 <= target <= 109
• Only one valid answer exists.

Follow-up: Can you come up with an algorithm that is less than O(n2) time complexity?

To solve the Two Sum problem in Java using a Solution class, we’ll follow these steps:

1. Define a Solution class with a method named twoSum.
2. Inside the twoSum method, create a hashmap to store elements and their indices.
3. Iterate through the array:
   • For each element, calculate the complement required to reach the target sum.
   • Check if the complement exists in the hashmap.
   • If found, return the indices of the current element and the complement.
   • If not found, add the current element and its index to the hashmap.
4. Handle edge cases:
   • If no solution is found, return an empty array or null (depending on the problem requirements).

Here’s the implementation:

import java.util.HashMap;

public class Solution {

    public int[] twoSum(int[] nums, int target) {
        // Create a hashmap to store elements and their indices
        HashMap<Integer, Integer> map = new HashMap<>();

        // Iterate through the array
        for (int i = 0; i < nums.length; i++) {
            int complement = target - nums[i];
            // Check if the complement exists in the hashmap
            if (map.containsKey(complement)) {
                // Return the indices of the current element and the complement
                return new int[]{map.get(complement), i};
            }
            // Add the current element and its index to the hashmap
            map.put(nums[i], i);
        }
        // If no solution is found, return an empty array or null
        return new int[]{};
    }

    public static void main(String[] args) {
        Solution solution = new Solution();
        
        // Test cases
        int[] nums1 = {2, 7, 11, 15};
        int target1 = 9;
        int[] result1 = solution.twoSum(nums1, target1);
        System.out.println("Example 1 Output: [" + result1[0] + ", " + result1[1] + "]");

        int[] nums2 = {3, 2, 4};
        int target2 = 6;
        int[] result2 = solution.twoSum(nums2, target2);
        System.out.println("Example 2 Output: [" + result2[0] + ", " + result2[1] + "]");

        int[] nums3 = {3, 3};
        int target3 = 6;
        int[] result3 = solution.twoSum(nums3, target3);
        System.out.println("Example 3 Output: [" + result3[0] + ", " + result3[1] + "]");
    }
}

This implementation provides a solution to the Two Sum problem with a time complexity of O(n), where n is the number of elements in the input array.

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